Mechanical and microstructural single-crystal Bauschinger effects: Observation of reversible plasticity in copper during bending

We study the Bauschinger effect on a bent and straightened micro-sized single-crystal copper beam (width: 8.64 μm; thickness: 7.05 μm) over three consecutive cycles. The reverse yield strengths (straightening step) are much smaller than those in forward loading (bending step). An upper bound estimate shows a load drop of 73% (1st cycle), 76% (2nd cycle) and 83% (3rd cycle) relative to the forward yield stress. Electron backscatter diffraction reveals a dramatic reduction in the bending-induced misorientation gradients upon load reversal (straightening), documenting an unexpected form of microstructure reversibility. The observed Bauschinger softening is interpreted in terms of two effects. The first consists of internal backstresses that support load reversal. They are created by polarized dislocation arrays that are accumulated during forward bending. The second effect is the reduced requirement to activate dislocation sources during reverse loading as the dislocations that were stored during bending did not participate much in cross-hardening and, hence, serve as mobile dislocations upon reverse loading. After straightening the misorientation gradients are largely removed but the non-polarized dislocations remain. We therefore introduce a revised terminology, namely the “mechanical Bauschinger effect” and the “microstructural Bauschinger effect”. The former term describes a yield stress drop and the latter one the degree of microstructure reversibility upon load path changes.